A vertical cavity surface emitting laser (VCSEL) is a type of semiconductor laser diode in which the optical beam is emitted in a direction normal to the top surface of a generally planar semiconductor structure. As illustrated in FIGS. 1-2, a conventional VCSEL 10 has a structure comprising a stack or number of layers that can be built up using photolithographic techniques. The structure can extend downwardly into a generally planar substrate stack 12, with a portion of the structure extending above the surface of substrate stack 12 including a P-metal layer 14 that is deposited on a raised or mesa region 16 of substrate stack 12. P-metal layer 14 represents the positive (P) electrical contact of the circuit that supplies current to VCSEL 10. P-metal layer 14 has a substantially ring-like or annular shape. In operation, VCSEL 10 emits light through the opening in the center of P-metal layer 14 substantially in the direction of the arrow 18. Note that arrow 18 is aligned along an optical axis 19 of VCSEL 10 that is normal to the planar semiconductor structure.
The remaining layers of the structure have similarly annular or circular shapes that are similarly symmetrically arranged with respect to optical axis 19, though this aspect is not shown in the enlarged cross-sectional view of FIG. 2. The layers are shown in generalized or schematic form in FIG. 2 for purposes of clarity. Also note that FIGS. 1-2 are not to scale. At the bottom of the structure, an N-metal layer 20 is deposited over a semiconductor (e.g., GaAs) substrate layer 22. N-metal layer 20 represents the negative (N) electrical contact of the circuit that supplies current to VCSEL 10. Above substrate layer 22 is an N-type lower distributed Bragg reflector (N-DBR) layer 24. Above N-DBR layer 24 is an active region 26 that can comprise one or more quantum wells. Above active region 26 is an oxide layer 28 having an annular shape that defines an oxide aperture 30. A P-type upper distributed Bragg reflector (P-DBR) layer 32 is disposed above oxide layer 28 and extends into oxide aperture 30. Oxide layer 28 helps direct electrical charge into active region 26. N-DBR layer 24 is sometimes referred to as the lower DBR of the VCSEL, and P-DBR layer 32 is sometimes referred to as the upper DBR of the VCSEL. An isolation implant layer 34 surrounds the periphery of P-DBR layer 32. Isolation implant layer 34 can be formed of P-DBR material in which ions are implanted to make the layer dielectric, so as to electrically insulate P-metal layer 14 from active region 26. A dielectric layer 36 between isolation implant layer 34 and P-metal layer 14 provides further electrical insulation. When a voltage is applied between P-metal layer 14 and N-metal layer 20, a current flows downwardly from P-metal layer 14 toward active region 26, causing photons to be emitted in the area of active region 26 within oxide aperture 30. The voltage is applied by coupling a source of high frequency electrical energy between a bondpad 35 and N-metal layer 20. A metal neck region 37 electrically connects bondpad 35 to P-metal layer 14.
The VCSEL described above is only one of several types known in the art. For example, in another common VCSEL configuration (not shown) the N-metal layer is on the top surface. A well can be etched beyond the active region, exposing the N-DBR region or N-type substrate, and the N-metal layer can be deposited over and in the well.
A VCSEL of the type described above can be modulated at high speeds (i.e., radio frequencies or RF) and used in a high-bandwidth optical data communication link. However, the modulation bandwidth is limited by several effects, including intrinsic properties of the optical-electrical conversion process, thermal effects, and electrical parasitic effects. The first of these effects relates to the VCSEL response rolling off as the VCSEL is driven above its resonant frequency. The second of these effects relates to the VCSEL response rolling off with an increase in temperature. The third effect relates to parasitic capacitances and inductances in the VCSEL that can cause frequency-dependent power transfer rolloff between the RF source and the VCSEL junction.